The present disclosure relates to developing devices used in image forming apparatuses employing electrophotography such as copiers, printers, facsimile machines, and multifunction peripherals incorporating their functions, and to image forming apparatuses provided with such a developing device. More particularly, the present disclosure relates to developing devices which supply two-component developer containing toner and carrier and which discharge excessive developer, and to image forming apparatuses provided with such a developing device.
In image forming apparatuses, a latent image formed on an image carrier composed of a photosensitive member or the like is developed by a developing device and visualized as a toner image. In one type of such developing devices, a two-component development system using two-component developer is adopted. This type of developing device stores in a developer container two-component developer (hereinafter also referred to simply as developer) containing carrier and toner, includes a developing roller for supplying developer to the image carrier, and includes a stirring/conveying member which supplies developer in the developer container, while conveying and stirring it, to the developing roller.
In the developing device using a two-component development system, while toner is consumed as development is performed, carrier remains in the developing device unconsumed. Thus, carrier which is stirred together with toner in the developer container degrades as the stirring frequency increases. As a result, charging performance of carrier with respect to toner gradually degrades.
To cope with that, a developing device is proposed which can prevent degradation in charging performance by supplying developer containing carrier to the developer container while discharging excessive developer.
Incidentally, the height of developer tends to decrease in a high humidity environment and to increase in a low humidity environment. This causes the weight of developer in the developer container to vary depending on the environment in which the image forming apparatus is used. As a result, when the environment changes from a high humidity one to a low humidity one, the discharge amount of developer may increase suddenly or when the environment changes from a low humidity one to a high humidity one, developing failure may occur due to an insufficient height of developer.
For example, a known developing device employs, as a method for sensing trouble such as degradation of toner in developer, a decline in the storage amount of developer, or a deterioration in the balance of the proportion between toner and carrier, one involving sensing, with two magnetic permeability sensors arranged in the developer container, trouble with developer based on the difference between the outputs of the two magnetic permeability sensors.